It is known that these pipelines are provided with double protection against corrosion, one being the protection afforded by a coating layer of plastic material and the other the protection provided by cathodic polarization of the steel of the pipeline with respect to the sea water, this cathodic polarization being obtained, for instance, by arranging zinc anodes at given intervals around the pipeline, the polarization taking place between the zinc bracelets and the portions of the steel pipe not protected by the coating as a result of the deterioration of this coating.
Although it is rather easy to detect and correct coating defects occurring before the positioning of the pipeline, no provisions are made to correct damage suffered during laying of the pipeline. Under these conditions the zinc bracelets are arranged at intervals based on a statistical evaluation of the defects.
It has been already proposed to control the cathodic protection after the positioning of the pipeline, either by measuring the potential of the pipe with respect to the surrounding medium, or by measuring the protection currents circulating either in the pipe or in the surrounding medium. The measurement of the protection current can not be done during the positioning because of the perturbations generated by the parasitic currents originating from the numerous electrical machines used during the positioning of a pipeline, such as the welding currents.
There are methods already known in electrochemistry to study the condition of the surface by measuring the impedance of a surface submerged in a conductive medium by using an electrode excited by an alternating-current generator of average frequency and a potential-measuring electrode located very closely to the studied surface. The impedance is a result of the relation between the measured potential and the excitation current delivered. Such a method of measurement is applicable only to an examination taking place very close to the surface (only a few tens of microns), due to the fact that when the distance is of the order of millimeter, the low resistivity of the medium especially when this medium is sea water has a tendency to equalize the potential between the excitation electrode and the substratum, so that it is no longer possible to measure the variations in impedance.